1. Field of the Invention
The present invention relates, in general, to flue gas desulfurization absorbers, and in particular, to a new and useful absorber arrangement having a gas inlet at the transition between a lower, large diameter tank section which contains a liquid slurry level, and an upper small diameter absorber section.
2. Description of the Related Art
Commercialization and development of high velocity absorbers is pursued because of the economic advantages they offer such as lower capital cost, less real estate requirements, shorter more compact absorbers, and improved SO2 removal efficiency. On the other hand, high velocity has some disadvantages such as increased resistance to gas flow and increased sensitivity of the system to changes in the hydraulic behavior of the gas and liquid phases. Physical model studies show that the gas velocity through the inlet of the absorber affects gas distribution in the absorber and affects the performance and behavior of the absorption zone and mist eliminator.
Regardless of the physical shape of the absorber, the resistance to gas flow is categorized as either useful resistance or parasitic resistance. Useful resistance is converted directly and entirely into scrubbing efficiency and participates in gas redistribution such as the absorption zone pressure drop. Parasitic resistance is expended to conduct the gas through the absorber confines without effective participation in the chemical process. The inlet and outlet resistances are good examples of this type of resistance. The use of turning vanes or other gas distribution devices is a simple solution to outlet resistance. However, the inlet pressure drop is not easy to reduce because it is a combination of the gas and the scrubbing liquid interaction throughout the absorber.
Traditional absorber inlets vary in shape and size but the shape of the inlets is basically the same. FIG. 1 shows the commonly offered inlet design (without protective awning). In this design, the liquid flowing off the absorber walls 12 or sprayed by nearby spray headers, falls into the inlet 14 and forms a solid growth at the wet/dry interface causing maldistribution and higher resistance. To overcome this problem and as shown in FIG. 2, protective intrusive awnings 16 were placed on top of the inlet 14 (see U.S. Pat. No. 5,281,402). The awning diverted the contact point (between the hot gas and the liquid curtain flow) from the vicinity of the inlet to the center of the absorber. Solids deposition is averted because gas humidification occurs in an area where there is minimum contact between the hot gas and the inlet flue surfaces. This design has been proven functional at the traditional gas velocities when the spray zone resistance is large enough to affect even distribution before the gas reached the mist eliminator further up in wall 12. As the gas velocity increases, however, the curtain resistance adds significantly to the overall system parasitic pressure drop and distortions to gas flow pattern becomes more critical.
While the liquid curtain is needed to humidify and help gas redistribution, it has two significant drawbacks. It significantly increases the inlet pressure drop compared to an inlet without an awning, and it distorts the flow pattern as the gas rises through the absorber.
In a new generation of high velocity absorbers, gas velocity is set between about 15 to about 20 feet per second. Minor distortion in the gas flow pattern results in localized high gas velocities approaching or exceeding the critical velocity of the mist eliminator and may result in functional failure of the mist elimination device.
To overcome the negative effects of high gas velocity in the inlet, one could increase the inlet""s flow area and limit the gas velocity to the conventional 3,000 feet per minute. This solution, while simple and practical, will result in a larger inlet aspect ratio (height to width) and increases the absorber height and inlet resistance. An increase in absorber height minimizes the advantages gained by high velocity scrubbing. Other options include advanced low pressure drop gas inlets for the new generation of high velocity absorbers, or the use of available means within the system to redistribute the gas flow without significant increase in inlet resistance.
The current industry inlet design uses the installation of the protective awning 16 on top of the inlet 14 to deflect the slurry away from the hot flue gas flow and prevents the deposition of solids at the wet/dry interface. However, at high absorber gas velocity, obstruction of the gas path by the high density liquid curtain deflects the gas to the sides causing an increase in gas velocity and possible distortion.
Model studies and operating experience teaches that at an absorber velocity between about 1 to about 12.5 feet per second, the current inlet designs provide good gas distribution across the absorber at or below about 3,000 feet per minute inlet velocity. The good gas distribution is provided partially by the resistance of the liquid curtain, falling off the awning to the entering gas. The primary function of the awning is to provide protection against inlet wetness and solids deposition and to provide ample resistance to slow down the entering gas, thus allowing the gas adequate time to redistribute itself across the absorber flow area. At an absorber gas velocity below 12.5 feet per second reasonable gas distortion in the absorber will not approach the critical failure velocity of the mist eliminator.
As the gas velocity increases above about 12.5 feet per second and approaches about 20 feet per second or more, the resistance of the liquid curtain falling off the awning becomes significant and magnifies the effects of gas flow distortion.
Several attempts were made to reduce the resistance of the awning first by introducing a new generation of non intrusive awnings. In these designs, the awning is removed from the inlet""s gas stream and placed above the inlet. See U.S. Pat. No. 5,403,523; 5,558,818 and 5,648,022. Each of these developments contributed to the reduction of the inlet""s parasitic pressure drop caused by the intrusion of the original awning into the gas flow path. These designs, however, added 3 feet to the height of the absorber and none of them totally eliminated the effect of the heavy density liquid curtain.
These prior efforts were steps in the right direction to reduce the inlet""s parasitic resistance, however, in every case the curtain resistance remained the same. Considering that one inch (water) of pressure drop is evaluated at approximately $1 million over the life of the power plant, reduction of the parasitic resistance of the absorber provides a significant competitive edge. Table 1 compares the pressure drop of an inlet with and without awning.
The present invention provides a combination of an advanced gas inlet with the sought protection against wet/dry zone solids deposits and reduces the liquid curtain to the same density observed in the inlet design without an awning. The new inlet does not promote the formation of thick high density liquid curtains in the gas path, thus reducing the parasitic pressure drop experienced with awning equipped inlets. The new design places the inlet in the transition between the large diameter tank and the small diameter absorption zone.
Successful application of this invention could possibly result in an evaluated cost of about $1,000,000 less than the current designs and over about $2,000,000 in savings over the original intrusive awning design.
The present invention is suitable for absorbers with flared tanks. The inlet is located in the transition between the tank and the absorber section. As a consequence of the location, the top plate of the inlet extends from about 1 to about 10 feet beyond the lower plate. This extension provides a natural protection to the inlet from the backflow of slurry into the hot gas zone. The smaller diameter absorption zone limits the area of falling liquid to the center of the tank leaving an annulus void of the falling slurry available for gas flow. This arrangement eliminates the need for an inlet bustle and utilizes the annulus to provide the gas with a path of low resistance. The falling liquid has an aspirating effect on the entering gas, thus, promoting gas distribution along the perimeter of the transition. The side walls of the inlet follows the contour of the conical transition. Side shields may be provided to protect the inlet from splashing, however, these side shields are expected to be added on a case by case basis. The smooth transition between the tank and the absorption zone provides means to contract the gas gently and evenly into the absorber section.
Accordingly, an object of the present invention is to provide, in an absorber having a lower large diameter tank section for receiving a slurry at a liquid level in the tank section, an upper small diameter absorber section where liquid scrubbing agent is mingled with flue gas from which impurities are to be absorbed, a low pressure inlet assembly comprising a transition structure between the tank section and the absorber section for closing a gas flow path and a liquid flow path between the tank section and the absorber section and an inlet housing connected to and communicating with the transition structure for inlet of flue gas into the transition structure between the tank section and the absorber section.
Another object of the present invention is to provide a low pressure inlet assembly for an absorber tower which is simple in design, rugged in construction and economical to manufacture.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.